The present invention relates to a friction reducing tool for use in well construction and servicing applications. In particular, the invention relates to a friction reducing tool suitable for use during drilling or casing installation procedures.
During exploration for oil, gas, geothermal activity, water or other naturally occurring substances, bores may be drilled to varying distances and can exceed several kilometres in length. Typically, it will be necessary to drill through layers of different formation, such as impermeable cap rock and permeable sandstone. Once a bore has been drilled it is necessary to isolate one formation from another to avoid problems associated with pressure differentials between the formations. Such isolation, known as zonal isolation, is achieved using casing or liner pipe which is cemented into the well bore. To obtain effective cementation it is necessary to centralise the casing or liner pipe in the well bore so that the cement sheath is of adequate thickness to provide suitable integrity.
Frequently, the length of the well bore is such that centralising tools become significantly worn on their trip to the bottom of the well. In an attempt to obviate this problem, tools have been developed to reduce friction caused by contact with the inside wall of the bore. For example, one known tool has groups of rollers positioned on the periphery of the tool.
It is often desirable to insert casing pipe into a bore where the cross sectional diameter of the bore is only marginally greater than the cross sectional diameter of the casing pipe. For example, casing pipe of 7 inch (18 cm) diameter may be required in a bore of 8.5 inch (22 cm) diameter. A small annular spacing will therefore tolerate only a correspondingly small distance between the contact surface of the rollers and the outer periphery of the friction reducing tool. This requires the use of small rollers which can have limited effectiveness in reducing friction. U.S. Pat. No. 5,778,976 discloses a friction reducing tool having rollers incorporated in radial support pedestals. GB-A-2241009 discloses a friction reducing tool having rollers in the form of discs.
Additionally, rollers of the type used in known friction reducing tools have axles which are limited in respect of cross-sectional diameters. Such axles may be prone to weakness and breakage. A further disadvantage of known roller tools is that cuttings or granular material in the bore can become jammed or wedged between the rollers and the pipe on which the roller tool is mounted.
It is therefore an object of the present invention to provide a friction reducing tool which overcomes the abovementioned disadvantages, or at least provides a useful alternative.
In one aspect of the invention there is provided a friction reducing tool having a generally tubular body and three or more groups of rotatable castors provided about the periphery of the body, the castors of each group being substantially aligned in a longitudinal direction, and each group of castors having at least one castor offset relative to at least one other castor of the same group, characterised in that each castor is rotatable about an axis extending substantially outwards from the surface of the body.
The at least one castor and the at least one other castor may be positioned on the tubular body so that the axis of rotation of one castor is parallel to the axis of rotation of the other castor and the two axes are diametrically offset relative to an axis parallel to the axis of the tubular body.
Alternatively or additionally, the at least one castor and the at least one other castor may be positioned on the tubular body so that the axis of rotation of one castor and the axis of rotation of the other castor are angled away from each other.
The at least one castor is preferably offset relative to the at least one other castor by an amount sufficient to enable contact of each castor with the inside wall of a bore when in use. Castors having parallel axes of rotation may, for example, be offset by 3-30 mm. Castors having angled axes of rotation may be angled away from each other by an angle of up to 50xc2x0 or more, but typically closer to 10xc2x0 to 20xc2x0.
Preferably each castor includes a rotatable disc and an axle. The outer surface of the disc is preferably convex in shape.
Preferably the tubular body contains an aperture for receiving an axle of a castor. It is preferred that the axle is fixed to the tubular body within the aperture and that the rotatable disc is free to rotate about the end of the axle protruding from the tubular body.
It is preferred that the three or more groups of castors are located substantially equidistant about the periphery of the tubular body. Preferably, there are five groups of castors.
While there may be any number of castors within one group of castors, preferably there is one or more pairs of complimentary castors offset to each other. In a preferred embodiment of the invention, each group of castors comprises a single pair of castors.
In a second aspect of the invention there is provided a method of using the friction reducing tool of the first aspect including fitting the tool to a pipe and running the pipe through the bore of a well.
In a preferred embodiment of the invention there is provided a friction reducing tool as described above fixed to a tubular section of a casing stand or drill string.